The internal combustion engine is widely used as a means of converting thermal energy into mechanical energy. It has been developed extensively over the past few decades, especially by motor vehicle manufacturers, into a compact, lightweight and efficient unit.
However, the principle of a reciprocating piston connected by means of a connecting rod to a crank shaft and constrained within a cylinder has inherent drawbacks. The piston, by the very nature of its function, has a significant mass and thus inertia. Consequently, the reciprocating motion causes vibration and also limits the maximum possible speed of rotation of the crank shaft.
The standard reciprocating engine, as used in automobiles, has both a relatively small mechanical efficiency and fuel efficiency. One reason for this is the short stroke of the engine. The limited amount of time possible for the power stroke leads to incomplete detonation. The inefficiency increases with engine speed because the time for combustion is correspondingly reduced.
Another disadvantage of the conventional piston engine results from valve overlap. Since both exhaust and intake valves are open at the same time, a proportion of the air/fuel mixture is exhausted unburnt. The thermal efficiency of the reciprocating engine is also considerably less than optimal. Detonation occurs before top dead center and so expansion of the gases causes the mixture to heat up rather than to provide work energy.
One further drawback is that the power stroke and the compression stroke are an identical length for any given piston. Since power is only derived on the exhaust or power stroke of the engine cycle, the efficiency of an engine could be improved by lengthening this part of the cycle. Although theoretically possible, the design of a conventional two stroke or four stroke engine does not lend itself to this.
Many attempts have been made to minimize or obviate these inherent disadvantages. The WANKEL (TM) or rotary piston engine is probably the most well-known of these, where a rotating piston is used to rotate a shaft and thus generate motive power. In this modification, the edges of a rotating piston open and close ports in a cylinder wall, so that the piston itself controls the "breathing" of the engine, without the aid of valves. The piston is substantially triangular in shape with convex sides and rotates in a cylinder whose internal cross-section has a substantially oval shape slightly constricted in the middle (epitrochoid). When the piston rotates, seals mounted at its three corners continuously sweep along the wall of the cylinder. The three enclosed spaces formed between the piston and the wall successively increase and decrease in size with each revolution. These variations in the spaces are utilized for drawing in the fuel-and-air mixture, for compressing this mixture, for combustion, and for discharging the burned gases. In this way, the full four-stroke working cycle is performed.
It will be appreciated that in the rotary piston engine there are no reciprocating masses which have to be alternatively accelerated and decelerated and the forces or inertia associated with the reciprocating motion are therefore obviated in this type of engine. As a result, higher speeds of rotation are theoretically possible.
However, one of the major problems in the construction of the rotary piston engine is the sealing of the three spaces in relation to one another. Intercommunication between these spaces would be detrimental to the proper functioning of the engine. This problem has been partly solved by means of a system of sealing strips.
However, the problem of wear and durability has only been partly resolved and as a consequence, these rotary engines have yet to find universal acceptance.
Many attempts have been made to improve the WANKEL engine, the most relevant of these known to the applicant being described in U.S. Pat. No. 4,401,070 (McCann). This describes an engine with a rotor and at least one vane extending slidably through the rotor in a transverse direction for rotation therewith. The vane has opposite ends extendible beyond the rotor, which itself rotates within a stator which has a hollow, cylindrical interior. The stator has opposite side walls with circumferentially extending depressions therein, the depressions of the opposite walls being staggered, causing transverse reciprocation of the vane as the rotor is rotated. The depressions are shaped to slidably receive the ends of each vane in sealing contact.
The stator is in effect two static housings which embrace the rotor and support it at either end of a rotor shaft. The housings contain two cavities formed in their ends into which the rotor plus sliding vanes fit.
This design relies upon a relatively complex series of ducts and holding volumes to transfer an aliquot of compressed gas from one side of a vane during the compression cycle to the reverse side of the same vane for the power stroke. This not only exacerbates the sealing problems inherent with this type of engine but requires complex machining during manufacture. It also means that cavities on each side of the rotor are used for compression and power strokes alternately.
It is therefore an object of the present invention to provide a new kind of internal combustion engine which does not suffer from these disadvantages.